This is a letter many English people worry about when speaking Spanish, because of its similarity in sound to the Spanish letter V. When I’m teaching and say a word with either B or V in it, I tend to notice people’s eyes fixed on my lips, to detect whether I’m differentiating between them or not! Sometimes of course ones mouth tends toward one sound or another depending on where it comes in a word, but essentially the letters B and V in modern standard European Spanish are identical in sound, and that sound is a bit like a lazy “B”. So, here’s how you do it. Practise making the English B sound first and you should notice a slight pushing out of air from tightly closed lips. Now imagine you’re far too exhausted to make that effort, so just close your lips very, very lightly and make the same sound without the little explosion of air.
Now remember that if you err on one side or the other, you will still be understood just the same. In other words, if you say “vivo” using the English “v” sound, or you pronounce the same word “bibo” with the English “b” sound, we still know you mean “I live” and a Spanish person will not detect the difference in the same way an English speaker will. This is because the consonants in general in Spanish have room for variation in a way that English consonant sounds do not.